(1) Field of the Invention
The present invention relates to a silicon nitride sintered body. More particularly, the present invention relates to a silicon nitride sintered body which is excellent in the oxidation resistance and mechanical strength at high temperatures.
(2) Description of the Prior Art
A composition comprising silicon nitride as the main component and an oxide of a rare earth element as the additive has been heretofore used as a silicon nitride material excellent in the strength at high temperatures. The reason is that if an oxide of a rare earth element (hereinafter referred to as "Re") is added, a compound of this oxide with silicon nitride formed in the grain boundary was a high melting point and hence, a sintered body obtained can have a high strength at high temperatures.
However, this silicon nitride sintered body comprising a rare earth element incorporated therein is oxidized when used in an oxidizing atmosphere for a long time, and a silicate of the rare earth element is formed, for example, by reaction of Re.sub.2 Si.sub.3 O.sub.3 N.sub.4 .fwdarw.Re.sub.2 Si.sub.2 O.sub.7. Since this silicate has a high melting point and a high glass transition temperature (Tg), a vitrious layer is not formed but the silicate takes the form of powder. Therefore, the silicate fails to form a protecting film selaing the surface layer of the sintered body.
Accordingly, oxidation advances even to the interior of the sintered body and the oxidation resistance at high temperatures of the silicon nitride sintered body is degraded.
Recently, Japanese Patent Application Laid-Open Specification No. 146981/84 has proposed a silicon nitride sintered body comprising oxides or oxynitrides of Sr, Mg, a rare earth element and Zr in amounts of 0.1 to 15% by weight, 0.1 to 15% by weight, 0.1 to 15% by weight and 0.1 to 11% by weight as Sr, Mg, the rare earth element and Zr, respectively, the balance being at least 70% by weight of Si.sub.3 N.sub.4. It is taught that if the Mg component is not contained, the strength (at room temperature) and the toughness at break (at room temperature) of the sintered body are degraded. However, the oxidation resistance or mechanical strength at high temperatures of the silicon nitride sintered body is not mentioned in this prior art at all.